Empty field

To go deeper into how Advanced Search Tool can help me in my quest of 
empty field, I built a test album with 4 elements in order to make some 
tests :

1. Image with EXIF section, and field "Make"="NIKON" (from DSLR)
2. Image with EXIF section, and "Make"=null (from ExifTool)
3. Image with EXIF section, but "Make" missing (saved from SMS/MMS by 
smartphone)
4. Image without EXIF section (build from scratch by GIMP)

I tried a lot of requests all based on the EXIF field "Make" (the 
manufacturer of the camera) and, at this step, it seems to me so strange...

1. Searching for the only album name => 4 images (as expected)
2. Searching for NIKON (second criteria in the main group : Photograph 
Information/The manufacturer of the camera) => 1 image (as expected)
3. Searching for all cases in the proposed listbox of Make criteria => 1 
image (versus 3 expected)
4. excluding NIKON  by adding a second search group with options 
AND/"None of these conditions are completed" and criteria "Make"=NIKON 
=> 1 image (versus 3 expected !!!)

What seems to me "out of MY sens" is that the case "Make = nothing" is 
not proposed in the relevant listbox of search number 3, while it is 
automatically excluded by the search number 4, WITHOUT ANY REQUEST FOR 
THAT (assuming I'd tick only fulfilled field values...) ...!

my questions :

- is this the way of working wanted for Digikam?

- if yes, how can I exclude the empty "Make" field values of my search 
(I never succeed in my attempts...)?


Hope my explanations (and my English) are clear enough!

Olivier